Question: Sequence with k

Comment on Sequence with k

Could you explain why we this approach is wrong
t10-t7 = t3
then set
tn = t3
36= 3^2 +3k
solve for K
k= 9
t25= 25^2 +9(25)
= 850
gmat-admin's picture

We can't conclude that t10 - t7 = t3 (as you have done in the first step of your solution)

To see why this isn't necessarily true, consider this sequence where term_n = n²

term_1 = 1
term_2 = 4
term_3 = 9
term_4 = 16
term_5 = 25
term_6 = 36
term_7 = 49
term_8 = 64
term_9 = 91
term_10 = 100

Notice that term_3 = 9
Also notice that term_10 - term_7 = 100 - 49 = 51
As you can see, term_10 - term_7 does not equal term_3

Cheers,
Brent

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